This invention relates to semiconductor devices, and more particularly to methods of making MOS integrated circuits.
In the local-oxidation or "LOCOS" process for making self-aligned, silicon-gate MOS integrated circuits, field oxide is grown at high temperature using nitride as an oxidation mask. A channel stop region is created beneath the field oxide by a P+ implant prior to oxidation. The P-type impurity from the channel stop implant encroaches upon the transistor channel areas during the high temperature oxidation. This creates a problem in geometric integrity, and can also raise the apparent threshold of the transistors, referred to as "narrow width effects," especially in high density devices where the channel widths are very small, perhaps one or two microns, as is the case for 1-megabit dynamic RAMs, for example.
It is the principal object of the invention to provide an improved process for making semiconductor integrated circuits, particularly high density MOS devices. Another object is to reduce the narrow width effect of encroachment of channel stop impurity in the manufacture of MOS integrated circuits. A further object is to provide improved isolation when using the local-oxidation process for making high density MOS devices.